Capacitive pressure sensor

ABSTRACT

A capacitive pressure sensor comprises a pair of conductive plates surrounding a compressible dielectric to form a capacitor. Changes in pressure create changes in the capacitance of the capacitor which in turn may be measured to determine the changes in pressure. The pressure sensor may be constructed to be temperature and centripetal force compensated so that it may be positioned in a tire. A further embodiment uses the conductive plates to form a radiating element for the sensor such that it may wirelessly communicate with a remote interrogator.

FIELD OF THE INVENTION

The present invention relates to a capacitive pressure sensor, andparticularly to a pressure sensor that is positioned in a tire andcommunicates information relating to the pressure to a remoteinterrogator.

BACKGROUND OF THE INVENTION

Every iteration of vehicle design results in greater complexity for thevehicle in an effort to improve functionality. Vehicles now monitor manyparameters that previously had to be addressed through routinemaintenance or expensive diagnostics. This monitoring is done by vehiclecontrollers. By providing this access to information about the vehicle,expectations about possibilities arise and this creates a demand formore information from which to synthesize still further parameters thatmay improve performance.

Tire pressure monitoring may be important since the pressure in a tiregoverns its proper operation and safety in use. For example, too littlepressure in a tire during its use can cause a tire to be damaged by theweight of a vehicle supported by the tire. Too much pressure can cause atire to rupture. Tire pressure is currently tested by hand-held deviceswhen the vehicle is stopped, and thus it is difficult to secure tirepressure information while the vehicle is operational.

In related areas, there has been an increase in the desire to trackgoods as they move through the manufacturing, distribution and retailprocesses. To that end, many goods are being equipped with radiofrequency identification tags (RFID). Tires are one such good that maybenefit from the use of a wireless communication device thatcommunicates information regarding the tire, such as a tire'sidentification, pressure, temperature, and other environmentalinformation. For example, tire pressure must be tested during themanufacturing process to ensure that the tire meets intended designspecifications. The tire pressure should also be within certain pressurelimits during use to avoid dangerous conditions. Knowledge of the tirepressure during the operation of a vehicle can be used to inform anoperator and/or vehicle system that a tire has a dangerous pressurecondition.

Thus, there remains a need for a pressure sensor associated with a tirethat can wirelessly communicate to a remote location.

SUMMARY OF THE INVENTION

The present invention provides a pressure sensor that measures pressurebased on a force placed on a dielectric material. The pressure sensormay be coupled to a wireless communication device that wirelesslycommunicates information relating to the pressure sensed to a remotelocation. In an exemplary embodiment, this pressure sensor may bepositioned in a vehicle tire and communicate wirelessly to a vehiclecontroller such that the vehicle controller is aware of the pressurewithin the tires.

At its simplest, the present invention comprises a pair of conductiveplates sandwiching a resilient, compressible dielectric to form acapacitor. As the pressure around the capacitor changes, the dimensionsof the compressible dielectric change, thereby changing the capacitance.This change in capacitance may be used to derive a pressure therefrom. Aradio frequency chip may communicate wirelessly to a remote location,conveying information about the pressure exerted on the capacitor.

In a first alternate embodiment, the capacitor's structure is changed tobe temperature compensated. A first contemplated structure includes atemperature measuring device that reports a temperature in conjunctionwith the reported pressure. From these two data points, the vehiclecontroller may derive a temperature compensated pressure. A secondcontemplated structure includes an element that changes shape accordingto known parameters according to temperature. The change in shape mayexert a force on the capacitor to provide a known effect on the shape ofthe capacitor to compensate for temperature effects on the capacitor.Several forms of structures are propounded which address thisembodiment.

In a second alternate embodiment, the capacitor's structure is changedto compensate for centripetal force generated by a tire rotating. Afirst contemplated structure includes orienting the capacitor'sconductive plates parallel to the direction of the rotational forces. Asecond contemplated structure comprises two capacitors secured to abridge element. One capacitor would expand under rotational forces andthe other capacitor would compress. By empirical testing, the ratio ofthese effects could be correlated to a rotational force offset.

In a third alternate embodiment, the capacitive plates form radiatingelements through which the chip may communicate. Contemplated structuresinclude forming a patch antenna with one plate of the capacitor orforming a dipole antenna on one surface of the dielectric with a radiofrequency chip positioned on that same surface.

Other embodiments may combine different elements from the first threeembodiments, such as a temperature and rotational force compensatingpressure sensor, a radiating temperature compensating pressure sensor orthe like.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 illustrates a vehicle with wheels mounted thereon;

FIG. 2 illustrates a schematic diagram of a control system associatedwith a vehicle and the control system's communication with sensors aboutthe vehicle;

FIG. 3 illustrates a first view of a pressure sensor according to oneembodiment of the present invention;

FIG. 4 illustrates a second view of the pressure sensor of FIG. 3;

FIG. 5 illustrates a second embodiment of a pressure sensor of thepresent invention;

FIG. 6 illustrates a top plan view of a third embodiment of a pressuresensor of the present invention;

FIG. 7 illustrates a side elevational view of the embodiment of FIG. 6;

FIG. 8 illustrates a third embodiment of a pressure sensor of thepresent invention in which an antenna is integrated into the pressuresensor;

FIG. 9 illustrates a top plan view of a fourth embodiment of thepressure sensor of the present invention;

FIG. 10 illustrates a side elevational view of the embodiment of FIG. 9;

FIG. 11 illustrates a permutation on the compressible dielectric usedwith the present invention;

FIG. 12 illustrates a fifth embodiment of the pressure sensor of thepresent invention that compensates for rotational forces;

FIG. 13 illustrates a sixth embodiment of the pressure sensor of thepresent invention that compensates for rotational forces; and

FIG. 14 illustrates a seventh embodiment of the pressure sensor of thepresent invention that compensates for temperature fluctuations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent information about the presentinvention to enable those skilled in the art to practice the inventionand illustrate the best mode of practicing the invention. Upon readingthe following description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the inventionand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

The present invention is, as indicated above, a capacitive pressuresensor that reflects changes in pressure by changes in capacitance. Itis specifically contemplated that this device will be used in a tire ofa vehicle, although other uses are possible. To assist in comprehension,a review of this context is in order.

The present invention is particularly contemplated as being used in avehicle, such as vehicle 10, illustrated in FIG. 1. More specifically,the present invention may be used in a wheel 12 of the vehicle 10. Thewheel 12 comprises a rim 14 and a tire 16 which encircles the rim 14thereby forming a cavity therebetween, as is well understood. Wheninflated, the tire 16 has pressurized air therewithin that may fluctuateand affect the performance of the vehicle 10. Further, the wheels 12rotate when the vehicle 10 is moving creating strong rotational forces.The vehicle 10 may comprise a control system 18 that monitors manyparameters about the vehicle 10, including fuel consumption, rate ofmovement, oil pressure, battery charge level, miles traveled, and thelike. Many of these functions are conventional. The present inventionadds monitoring pressure within the wheels 12 to the other parametersmonitored. Additionally, the present invention may be used during themanufacturing of the tires, and specifically during the pressure testingthat is performed during the manufacturing. Armed with information aboutthe tire pressure, vehicle operators may make informed decisions aboutsecuring service or the like.

While measuring the pressure in the wheel 12 is a particularlycontemplated use of the present invention, other uses of the pressuresensor of the present invention are also contemplated. Further, whilethe present disclosure explicitly details techniques to compensate forforces present in an operational wheel 12, other force compensatingstructures appropriate to other environments could be used as well.

As illustrated in FIG. 2, the vehicle control system 18 may be connectedto a pressure sensor 20 (and optionally a temperature sensor 22) via awireless communication link, such as through an interrogator 24. Thevehicle control system 18 may comprise any type of electronic circuitryincluding a microprocessor microcontroller the like associated withsupporting circuitry and software. Alternatively, a special purposeintegrated circuit or the like could also be used. The vehicle controlsystem 18 may comprise a memory (not shown explicitly) executed programsoftware. Other sensors 26 may also report sensed data to the vehiclecontrol system 18. Such other sensors 26 may comprise vehicle tankmonitoring sensors, oil pressure sensors, engine revolution sensors,battery charge sensors, rate of movement sensors, and the like.

The vehicle control system 18 may further be operationally connected toan output 28. The output 28 may take a number of different formssimultaneously. For example, tachometers, speedometers, odometers, andthe like all form outputs for the other sensors 26. The output 28 mayalso comprise a data port (not shown explicitly) to which anothercomputing device is connected and through which data may pass so thatthe other computing device may run diagnostics or the like on the dataso passed. A more detailed exploration of this material may be found incommonly owned U.S. patent application Ser. No. 10/012,206 filed Oct.29, 2001, which is hereby incorporated by reference in its entirety.

The interrogator 24 may comprise its own internal energy source, such asa battery, or the interrogator 24 may be powered from the vehicle engineor car battery (not shown). The interrogator 24 communicates by emittinga signal modulated by interrogation communication electronics through aninterrogation antenna (none shown). The interrogation antenna may be anytype of antenna that can radiate a signal through a field so that areception device can receive such signal through its own antenna. Thefield may be magnetic, electric, or electromagnetic. The signal may be amessage containing information and/or a specific request to perform atask or communicate back information.

Other arrangements for the control system 18 of the vehicle 10 are alsopossible. The connections between the components may be wire-based orwireless. Other functionalities such as global positioning receivers andthe like are also possible. Such other functionalities lie beyond thescope of the present invention and are not germane thereto. Those ofordinary skill in the vehicular art can readily assemble comparable orequivalent systems based on the teachings herein.

The pressure sensor 20 is illustrated, in an exemplary embodiment, inFIGS. 3 and 4. FIG. 3 is a combination of illustration and schematicdiagram in that the pressure sensor 20 is shown partially in a frontperspective view for a capacitor 30 and a block format for capacitancemeter 31. The pressure sensor 20 may further comprise connection leads21 connecting the capacitor 30 to the capacitance meter 31, and thenceto an RF communication circuit 32. When a force is placed on thepressure sensor 20, the capacitor 30 has its shape altered, therebyaltering the capacitance thereof. The capacitance of the capacitor 30may be correlated to pressure as described in greater detail below.

The capacitor 30 comprises a resilient, compressible dielectric 34, suchas a foam material. Appropriate materials include, but are not limitedto: silicone foam material, rubber material, synthetic rubber material,neoprene, polyurethane foam, and Polytetrafluoroethylene (PTFE) foam.The requirements for the dielectric 34 are that it be resilient,generally non-conductive, and not lose its elasticity after repeatedcompressions and expansions. In a more preferred embodiment, thedielectric 34 comprises a plurality of air pockets therein, such as in asilicone foam material. Positioned contiguously on either side of thedielectric 34, and sandwiching the dielectric 34 therebetween areconductive layers 36 and 38. Contiguous, as used herein, means beingproximate to or in actual contact. Sandwich, as used herein, means toinsert or enclose between at least two things of another quality orcharacter. An adhesive (not shown) may be used to keep the conductivelayers 36, 38 in close contact with the compressible dielectric 34 aswould be well understood. Conductive layers 36, 38 may be comprised ofmetalized polymers, conductive inks printed on the surface of thedielectric 34, metallic foils or the like. The arrangement of twoconductive layers 36, 38 sandwiching the dielectric 34 therebetweenforms the capacitor 30. As pressure changes around the capacitor 30, thedielectric 34 compresses and expands, thereby changing the capacitanceof the capacitor 30. The capacitance is measured by the capacitancemeter 31. This change in capacitance is a function of the pressure, andwith suitable empirical tests, can be used to provide an indication ofthe pressure affecting the pressure sensor 20. An exemplary capacitancemeter 31 comprises the Wavtek HD115B.

In an exemplary embodiment, the conductive layers 36, 38 are copper andapproximately 20 mm×10 mm. The dielectric 34 comprises a 1.5 mm thicksilicone foam rubber. Preliminary testing of such a device indicatesthat capacitance is a linear or near-linear function of pressure andthus pressure is easily derived from the measurements of the capacitancemeter 31. Capacitance meter 31 may have approximately 6 pF ofcapacitance at its input, which may be deducted from its measurementswhen determining pressure. Additionally, the capacitance meter 31 may beintegrated into the RF communication circuit 32.

The RF communication circuit 32 may comprise a control system,communication electronics, memory, and an antenna for receiving andtransmitting modulated radio-frequency signals. Various embodiments willuse some of these components in conjunction with the pressure sensor 20and thus a detailed analysis of what component serves what function willbe presented below. The following overview discusses a genericarrangement such as may be further developed by those skilled in theart. The control system may be any type of circuitry or processor thatreceives and processes information received by the communicationelectronics, including a microprocessor or microcontroller. The memorymay store information relating to the pressure measurements from thepressure sensor 20. The memory may be electronic memory, such as randomaccess memory (RAM), read-only memory (ROM), flash memory, diode, etc.or the memory could be mechanical such as a switch or dipswitch.Exemplary RF communications circuits may be found in U.S. Pat. No.5,347,280, which is hereby incorporated by reference in its entirety.

Temperature sensor 22 may be contained within the RF communicationcircuit 32 or external thereto as needed or desired. Exemplarytemperature sensors 22 may comprise anemometer, semiconductor devices, achemical device, thermistors or those disclosed in U.S. Pat. Nos.6,299,349; 6,272,936; 5,959,524, and 5,961,215, all of which are herebyincorporated by reference in their entirety. Note that the temperaturesensor 22 is an optional feature and may be omitted.

It is possible to integrate the capacitance meter 31, the temperaturesensor 22, an the RF communication circuit 32 into a single device orchip. Alternatively, these elements may remain discrete components ifneeded or desired.

As a further option, which is not central to the present invention, theRF communication circuit may further comprise an RFID chip. Reference ismade to the previously incorporated '206 application for a more lengthydiscussion of how such a chip works.

Alternate Embodiments

In the embodiment of FIGS. 3 and 4, if the capacitor 30 is positioned ona planar substrate (such as a printed circuit board) with firstconductive layer 36 on the substrate, it is necessary to make aconnection to the second conductive layer 38, and this connection mustaccommodate the changing position of the second conductive layer 38 aspressure changes. This adds complexity to the manufacturing process.

FIG. 5 presents one technique by which this difficulty may be addressed.Specifically, a capacitor 30A comprises a compressible dielectric 34A.The first conductive layer comprises two conductive co-planar plates 40,42. In this arrangement, the dielectric 34A and the second conductivelayer 38A form a bridge over two contacts (plates 40, 42) so that nocontact between the capacitive meter 31 and the second conductive layer38A is needed. Instead, the capacitive meter 31 provides connectionleads 21 to plates 40, 42. The capacitor 30A behaves as two parallelplate capacitors in series. The measured capacitance will change as afunction of pressure as previously described and pressure can be derivedfrom the measured capacitance.

FIGS. 6 and 7 illustrate a third embodiment of the present invention,namely one with an interdigital structure. Capacitor 30B comprises asubstrate 44 and a compressible dielectric 34B. The substrate 44 may bea printed circuit board or other generally non-conductive material. Thefirst conductive layer 36B and the second conductive layer 38B areformed as interdigital fingers and are connected directly to a sensorchip 50. The compressible dielectric 34B is positioned on top of thefirst and second conductive layers 36B, 38B, sandwiching the conductivelayers between the compressible dielectric 34B and the substrate 44. Thesensor chip 50 may include the capacitance meter 31, the RFcommunication circuit 32 and the like as needed or desired. Thecapacitance that is measured is a function of the net dielectricconstant seen by the interdigital fingers 36B, 38B. The electric fieldis partially in the compressible dielectric 34B and partially in thesubstrate 44. As the compressible dielectric 34B expands as a result ofdecreased pressure, the average dielectric changes, resulting in achange in measured capacitance. Likewise, as the compressible dielectric34B compresses as a result of increased pressure, the average dielectricchanges again, resulting in a change in measured capacitance.

An alternate embodiment of the interdigital structure of FIGS. 6 and 7comprises using a compressible dielectric over the interdigital fingersand a higher dielectric constant non-compressible material on top of thefoam (not shown). As the foam expands and contracts, the higherdielectric material moves relative to the interdigital structures, thuscausing a potentially larger change in capacitance than otherwise.

FIG. 8 illustrates a fourth embodiment of the capacitor 30C. Capacitor30C comprises a compressible dielectric 34C, a first conductive layer36C, a sensor chip 50C, and first and second radiating elements 52, 54,respectively. Radiating elements 52, 54, are positioned on either sideof the sensor chip 50C, and are generally co-planar. First conductivelayer 36C may function as a ground plane. At comparatively low and DCfrequencies, the structure of the capacitor 30C can be considered acapacitor. However, at higher frequencies, the conductive structuresassociated with radiating elements 52, 54 become a significant fractionof a wavelength, and the areas of conductive material can also act asantenna elements. Specifically, a dipole antenna 56 may be formed byradiating elements 52, 54. For further information about such radiatingstructures, reference is made to U.S. patent application Ser. No.09/618,505, filed Jul. 18, 2000 and U.S. patent application Ser. No.09/678,271, filed Oct. 3, 2000, both of which are hereby incorporated byreference in their entirety. Specifically, these applications deal withsymmetric (shown in FIG. 8) and asymmetric antenna structures (notshown).

In use, as the pressure varies around capacitor 30C, the compressibledielectric 34C expands and contracts. There are effectively two parallelplate capacitors in series connected to the sensor chip 50C, formed bythe radiating elements 52, 54 and the first conductive layer 36C. Sensorchip 50C may include the capacitance meter 31 and measure thecapacitance. The change in the compressible dielectric 34C may changethe tuning of the antenna 56 so formed at high frequencies. However,with good antenna design, this can be managed.

Other antenna structures are also contemplated, including a half wavepatch antenna, a monopole antenna, a loop antenna or the like.

One of the reasons that a foam material is preferred for thecompressible dielectric is that foam has bubbles of air trappedtherewithin that change size with changes in pressure. While air isparticularly contemplated, other gases or mixtures of gases may also betrapped within the material, Changing the dimensions or characteristicsof the compressible dielectric changes how it responds to the pressureand may provide assistance in achieving linearity of the functionbetween pressure and capacitance. For example, FIGS. 9 and 10 illustratean alternate shape for the compressible dielectric 34D in the capacitor30D. Specifically, the compressible dielectric 34D may be formed morelike a puck rather than the rectilinear shapes previously illustrated.Also, while alluded to elsewhere in the specification, it is alsopossible to have conductive layers 36D, 38D that are sized differentlyfrom the compressible dielectric 34D.

FIG. 11 illustrates another technique by which the shape of thecompressible dielectric 34 may be manipulated. Specifically,compressible dielectric 34 delimits one or more apertures 57 and/ordepressions 58. Either of these delimited structures exposes more areato external pressure, changes the mechanical properties of thecompressible dielectric 34, and thus helps customize the compressibledielectric 34 to the needs of the designer. Note that while theapertures 57 and depressions 58 are illustrated as cylindrical voids,other shapes are possible if needed or desired by the designer. Oneadvantage of these structures comes when the dielectric 34 with theapertures 57 is sandwiched between two conductive layers 36, 38 thatalso have apertures (not shown). If the conductive layers 36, 38 areconstructed so that they do not move with pressure (making it easier toconnect them to sensor chip 50) there is still a change in capacitanceas the walls of the apertures 57 will bulge inwards and outwardsdepending on the applied pressure. This changes the average density ofthe dielectric 34, thereby changing the dielectric constant and changingthe capacitance thereby. When the dielectric 34 of FIG. 11 is used withmovable conductive layers 36, 38, greater response to pressure may beachieved.

As should be appreciated, the differently sized and configureddielectrics 34 may be used with different capacitors 30. Thus, apuck-shaped dielectric 34 could be used with antenna 56, the twocapacitor arrangement of FIG. 5 or the like as needed or desired.

As would be expected, if the pressure sensor 20 is mounted in a wheel 12of a vehicle 10, the pressure sensor 20 is exposed to high levels ofrotational force as the wheel 12 turns during the driving of the vehicle10. This rotational force will distort the shape of the compressibledielectric 34.

Before discussing this further, a few definitions are in order.“Centripetal force,” as used herein, means the force that is necessaryto keep an object moving in a circular path and that is directed inwardtoward the center of rotation. “Centrifugal force,” as used herein,means 1) the force that tends to impel a thing or parts of a thingoutward from a center of rotation, and 2) the force that an objectmoving along a circular path exerts on the body constraining the objectand that acts outwardly away from the center of rotation. Bothcentripetal and centrifugal forces are defined herein as “rotationalforces.”

In a first technique to address these rotational forces, the vehiclecontroller 18 may compensate for rotational forces with software. Thevehicle controller 18 may pull information from the other sensor 26 todetermine the vehicle speed, and from this, derive a rotational speed ofthe wheel 12. A look-up table, formula, or algorithm stored in memory orthe like may be used with the rotational speed information,cross-referenced with the reported capacitance sensed to derive orcalculate the compensated pressure.

Alternatively, the pressure sensor 20 may be modified in structure tocompensate for the rotational forces. FIG. 12 illustrates one suchcompensation scheme wherein capacitor 30E is mounted parallel to therotational force axis 60 and perpendicular to the rim 14 (not shown inFIG. 12). Capacitor 30E comprises a compressible dielectric 34E, with afirst conductive layer 36E and a second conductive layer formed by twoL-shaped conductive members 62, 64. L-shaped conductive members 62, 64function as connection leads 21 in this embodiment. In this manner, asthe rotational acceleration increases, it will apply a shearing force onthe dielectric 34E. However, if the dielectric is relatively thin, andthe first conductive layer 36E of a low mass, the change in overallcapacitance will be small in comparison to the changes in capacitancedue to pressure changes.

A second rotational force compensation scheme is illustrated in FIG. 13,wherein a capacitor 30F comprises a first compressible dielectric 34F, asecond compressible dielectric 34F′, a first conductive layer 66 and asecond conductive layer 68, each positioned contiguous their respectivedielectric layers 34, and leads 70, 72. Leads 70, 72 are generallyS-shaped, conductive, and substantially rigid. This structure createstwo double series capacitors in effect. When an acceleration force 60 isapplied, the dielectric 34F is compressed (increasing the capacitancefor those two capacitors), and the dielectric 34F′ expands as it isstretched (decreasing the capacitance for those two capacitors). Thiscan result in a net zero change in capacitance due to rotational forces,effectively compensating for the rotational forces. As might beexpected, this structure too could be mounted parallel to the rotationalforces like the embodiment of FIG. 12.

In another embodiment, two sensors 20 may be used. One sensor 20 may bemore susceptible to rotational forces than the other. As rotationalforce increases, the difference between the two sensors may be used todetermine the rotational force exerted, and from that determine acompensation scheme to apply to the capacitance sensed by thecapacitance meter 31.

Still other rotational force compensators are contemplated, such as acoaxial tube, or any other pair of shapes which helps negate or minimizerotational force affects on the capacitors.

In the event that it is considered undesirable to include a specifictemperature measuring element 22, it is possible to compensate fortemperature effects through an appropriate structure. In a firsttechnique, ceramic elements with dielectric constants that change withtemperature are known. Specifically, some materials have dielectricconstants that increase as temperature increases, and some materialshave dielectric constants that decrease as temperature increases. Thesecan either be included as a layer in the compressible dielectric 34,compensating the overall capacitance, or as a separate capacitor placedin parallel with the capacitor 30 so that the total capacitance staysconstant.

In a second technique of temperature compensation, layered plasticsand/or metals can make structures that provide a relatively large degreeof mechanical movement with temperature. As illustrated in FIG. 14, suchmovement may be used to move the parallel plates of a capacitor.Specifically, capacitor 30G comprises a compressible dielectric 34G, afirst conductive layer 36G, a second conductive layer 38G, and atemperature compensating member 74. As the temperature changes, member74 bends according to its mechanical properties, which moves firstconductive layer 36G on compressible dielectric 34G such that eithermore or less overlappage be existent between first and second conductivelayers 36G, 38G.

Alternatively, first and second conductive layers 36G, 38G may be formedof temperature-induced shape changing conductive materials such that theconductive layers 36G, 38G move of their own accord in the presence oftemperature changes providing the desired compensation for temperaturechanges.

In still another embodiment, similar to that shown in FIG. 13, firstdielectric layer 34F could have a temperature coefficient in onedirection and second dielectric layer 34F′ could have a temperaturecoefficient in the other direction. Thus, as temperature increases, onedielectric layer 34F or 34F′ would contract (increasing capacitance) andthe other dielectric layer 34F or 34F′ would expand (decreasingcapacitance). Through measurements, these could offset one another suchthat only the effect due to pressure was being measured by thecapacitance meter 31. Other arrangements of capacitors in parallel withsimilarly opposite temperature compensating dielectrics 34 may also beused.

In another embodiment (not shown explicitly), the sensor chip 50 ispositioned next to the dielectric 34, and bond pads are used as both theconductive layers 36, 38 as well as connection leads 21. Given the smallsize of this structure, if more capacitance is required, the dielectric34 may be doped with a ceramic element. This ceramic element could alsoprovide temperature compensation as described above. For example, BariumTitanate (BaTiO3), which has a dielectric constant (Er) of 38.6 in itspure state, can be modified to have both positive and negativetemperature coefficient of dielectric constant (Tc, usually expressed inpart per million, ppm) by the inclusion of other metal oxides, such aszinc and strontium. Er is another symbol for the dielectric constant andTc is another symbol for temperature coefficient of the dielectricconstant in parts per million.

Loading the dielectric 34 with a ceramic material could be done in anyof the embodiments illustrated, and may be done to make the capacitanceand the capacitance change larger. This may reduce measurement errors.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

1. A pressure sensor, comprising: a first conductive layer; acompressible dielectric contiguous said first conductive layer; a secondconductive layer contiguous said compressible dielectric, wherein saidfirst and second conductive layers form a capacitor; and said capacitoradapted to reflect a capacitance in relation to pressure changes on saidcompressible dielectric.
 2. The pressure sensor of claim 1, wherein saidcompressible dielectric comprises a resilient material.
 3. The pressuresensor of claim 2, wherein said resilient material comprises a materialselected from the group consisting of: silicone foam material; foammaterial; rubber material; synthetic rubber material; neoprene;polyurethane foam; and PTFE foam.
 4. The pressure sensor of claim 1,wherein said first conductive layer comprises two conductive plates. 5.The pressure sensor of claim 4, wherein said two conductive plates areco-planar.
 6. The pressure sensor of claim 1, further comprising aremote communication device operatively associated with said capacitorfor sending pressure related information to a remote location.
 7. Thepressure sensor of claim 6, wherein said remote communication device isco-planar with said first conductive layer.
 8. The pressure sensor ofclaim 7, wherein said first conductive layer comprises two radiatingelements operative to radiate as an antenna for said remotecommunication device.
 9. The pressure sensor of claim 6, wherein saidremote communication device is adapted to respond to interrogations fromthe remote location.
 10. The pressure sensor of claim 1, furthercomprising a temperature measuring element.
 11. The pressure sensor ofclaim 10, further comprising a remote communication device and whereinsaid temperature measuring element communicates temperature informationto said remote communication device.
 12. The pressure sensor of claim 1,further comprising a temperature compensating element associated withsaid capacitor.
 13. The pressure sensor of claim 12, wherein saidtemperature compensating element comprises a ceramic element.
 14. Thepressure sensor of claim 13, wherein said ceramic element is disposedwithin said compressible dielectric.
 15. The pressure sensor of claim13, wherein said ceramic element comprises a second dielectricsandwiched between a third and fourth conductive layer, forming a secondcapacitor in parallel with said capacitor.
 16. The pressure sensor ofclaim 12, wherein said temperature compensating element comprises alayered plastic and metal stricture.
 17. The pressure sensor of claim 1,wherein said conductive layers are formed from a conductive ink printedon said compressible material.
 18. The pressure sensor of claim 1,wherein said conductive layers comprise metal foils.
 19. The pressuresensor of claim 1, wherein said conductive layers comprise metalizedpolymers.
 20. The pressure sensor of claim 10, wherein said temperaturemeasuring element is selected from the group consisting of: athermistor, an anemometer, a semiconductor type, and a chemical device.21. The pressure sensor of claim 1, wherein said first and secondconductive layers sandwich said compressible dielectric.
 22. Thepressure sensor of claim 1, wherein said first and second conductivelayers are substantially co-planar and form an interdigital structure.23-61. (canceled)
 62. A method of sensing pressure in a wheel comprisinga tire and a rim, comprising: forming a pressure sensor from acompressible material and a pair of conductive layers, thereby creatinga capacitance; placing the pressure sensor in the tire; and determiningpressure changes by changes in the compressible foam which affect thecapacitance of the pressure sensor.
 63. The method of claim 62 furthercomprising mounting the pressure sensor on the rim.
 64. The method ofclaim 62 further comprising associating a remote communication devicewith the pressure sensor.
 65. The method of claim 62 further comprisingmeasuring a temperature proximate the pressure sensor.
 66. The method ofclaim 62 further comprising compensating for rotational forces acting onthe pressure sensor.
 67. The method of claim 62 further comprisingcompensating for temperature changes proximate the pressure sensor. 68.The method of claim 64 further comprising communicating with a remotelocation through the remote communication device.
 69. The method ofclaim 67 wherein compensating for temperature changes comprisesinserting a ceramic material in the compressible dielectric.
 70. Amethod of sensing pressure in a tire on a vehicle, comprising: forming apressure sensor from a compressible material and a pair of conductivelayers; associating a remote communication device with the pressuresensor; placing the pressure sensor in a tire; determining pressurechanges by changes in the compressible material which affect thecapacitance of the pressure sensor; and interrogating the remotecommunication device to convey information to the vehicle's controlsystem about pressure changes in the tire. 71-75. (canceled)